Acidic media enables oxygen-tolerant electrosynthesis of multicarbon products from simulated flue gas

Renewable electricity powered electrochemical CO2 reduction (CO2R) offers a valuable method to close the carbon cycle and reduce our overreliance on fossil fuels. However, high purity CO2 is usually required as feedstock, which potentially decreases the feasibility and economic viability of the process. Direct conversion of flue gas is an attractive option but is challenging due to the low CO2 concentration and the presence of O2 impurities. As a result, up to 99% of the applied current can be lost towards the undesired oxygen reduction reaction (ORR). Here, we show that acidic electrolyte can significantly suppress ORR on Cu, enabling generation of multicarbon products from simulated flue gas. Using a composite Cu and carbon supported single-atom Ni tandem electrocatalyst, we achieved a multicarbon Faradaic efficiency of 46.5% at 200 mA cm-2, which is ~20 times higher than bare Cu under alkaline conditions. We also demonstrate stable performance for 24 h with a multicarbon product full-cell energy efficiency of 14.6%. Strikingly, this result is comparable to previously reported acidic CO2R systems using pure CO2. Our findings demonstrate a potential pathway towards designing efficient electrolyzers for direct conversion of flue gas to value-added chemicals and fuels.

2. More information should be provided regarding the computational methods, such as why the top two layers of Cu (111) were chosen to move freely, the criteria for selecting K-points, and whether spin polarization was considered.
3. Did the authors test the performance of Cu PTFE in 1 M Cs+ and 3 M Cs+ solutions for CO2R with flue gas? 4. In Figure S20, the authors claim that the CO2R performance of Cu PTFE/Ni-N4 surpasses that of Cu PTFE/Ni-N4-1 and Cu PTFE/Ni-N4-2.What will happen to the performance if more layers are added to the catalyst system? 5. Please elucidate the procedure employed by the authors to determine the area/proportion of *COhollow, *CObridge, and *COatop, as well as how they validated the ratio of *CO(hollow + bridge) to *COatop?Is this method an original development, or has it been previously established?If this is not a new method, kindly provide additional details and cite the relevant research articles.
6. Authors should check through the manuscript for mistakes, for example there are some errors in subscripts/superscripts.
Reviewer #4 (Remarks to the Author): The manuscript entitled "Acidic media enables Oxygen-tolerant Electrosynthesis of Multicarbon Products from Simulated Flue Gas" describes the discovery that the choice of electrolyte and catalyst design can be used to enable oxygen-tolerant production of C2+ products in simulated flue gas.The work presented in the manuscript has some novelties and the idea is interesting and relevant to the needs of the current CO2 reduction experiments.
The manuscript is well organized, however, there are still some issues that need to be addressed.

Subject comments
1.In the abstract and introduction section, abbreviations Ni-N4 need to be explained.
2. Although the author claims the catholyte is acidic (0.05M H2 SO4 + 0.5 M K2SO4 or 1.5 M Cs2SO4), it would be nice to know about the operating pH conditions throughout the experiment.S11, shows that the overall FE is over 100%.Why those discrepancies? 5.The author produced the EIS spectrum in Figure S18, but there is no significant change in the Rct value of the three catalysts to claim that the Ni-N4 catalyst possesses faster kinetics.Thus, the claim is ambiguous.

The XPS spectrum in
6.The authors performed DFT simulations on ORR activity, however in acidic conditions a DFT calculation about CO2RR with hydrogen evolution path will give more insights.
7. What is the practical relevance of this single atom catalyst?How feasible is it to upscale it and bring to practical use beyond the lab scale study?
8. What about testing with real flue gas which will give a more realistic case?
We thank the editor for handling our manuscript and the reviewers for their comments, which we have used to improve the quality of the work.Changes within the manuscript and supporting information are highlighted in yellow.Below is a response to the reviewer comments, which are written in blue font.
Reviewer #1 (Remarks to the Author): In this work, the authors reported that direct conversion of CO2 in simulated flue gas to C2+ products with Cu PTFE/Ni-N4 composite catalyst, where obtained a total C2+ FE of 46.5% at a current density of 200 mA cm-2 with acidic electrolyte for direct conversion of CO2 in simulated flue gas.As we all know, C2+ products will be produced by copper catalyst on electrochemical CO2 reduction reaction.In the other hand, many works has been reported that CO formation with Ni-based catalyst on electrochemical CO2 reduction reaction (Inorg.Chem. Front., 2019,6, 1729-1734;Angew. Chem. Int. Ed. 2020, 59, 4043-4050;Dalton Trans., 2023, 52, 928-935;Appl. Catal. B 2020, 271, 118929).In addition, tandem catalysis on electrochemical CO2 reduction reaction (from CO2 to CO, and then to C2+) by different catalysts or catalytic sites have also been reported by many works (J. Energy Chem. 2022, 70, 219-223;Nano Res. 2021, 14, 4471-4486;Angew. Chem. Int. Ed. 2021, 60, 25485-25492), including the work from the same group (Angew.Chem. Int. Ed. 2023, 62, e202308782).Only changing electrocatalyst (Cu-PTFE and Ni-N-C) is not novel in the same tandem reaction condition.No unique scientific insights were provided in this work.In general, the enhanced catalytic performance by constructing the Cu PTFE/Ni-Ni4 composite catalysts in this manuscript is within expected.Only simple combination of two well-known electrocatalysts does not attach the level of this strict scientific journal.

Response
We are thankful to Reviewer #1 for taking the time to evaluate our manuscript.
We would like to emphasize that the main focus and novelty of the work is on identifying an electrolyte composition and electrocatalyst system combination that can enable the generation of multicarbon (C2+) products directly from simulated flue gas.In this process, we needed to solve the two main challenges involved, which is the low CO2 concentration (15%) and the competing oxygen reduction reaction.For the first time, we have successfully demonstrated in this work that acidic electrolyte in combination with a Cu PTFE/Ni-N4 tandem electrocatalyst system can solve these challenges.This is the main novelty and focus of this work.
We also emphasize that the tandem electrocatalyst design for conventional electrochemical CO2 reduction with pure CO2 feedstock is not the main focus or novelty of this work.As the reviewer correctly states, there has already been a significant amount of literature on such tandem electrocatalyst systems.However, we note that all these studies were conducted in alkaline and neutral media.Therefore, we did not know beforehand what kind of tandem electrocatalyst configuration would exhibit high FE towards C2+ products in acidic media.
Although the reviewer mentions that "the enhanced catalytic performance by constructing the Cu PTFE/Ni-Ni4 composite catalysts in this manuscript is within expected", we found this not to be true.In our early experimental trials, we empirically found that quite a few of the tandem electrocatalyst systems that work well in alkaline/neutral media do not actually perform very well in acidic media.This issue is not previously well-known and hence is not something that can be said to be "within expected".This is the main reason why it was necessary in the first part of our manuscript to first verify that the tandem Cu PTFE/Ni-N4 electrocatalyst system that we designed can indeed exhibit a high FE towards C2+ products in acidic media using pure CO2 feedstock.In fact our C2+ FE of 82.4% is quite comparable to reported state-of-the-art acidic electrochemical CO2 reduction systems using pure CO2 feedstock.
Only after we had developed this electrocatalyst system, could we then move on to the next step, which is the main focus and novelty of this work, where we successfully demonstrated the use of this electrocatalyst system in combination with acidic electrolyte to generate C2+ products from simulated flue gas with FE of 46.5% at 200 mA cm -2 .However, we acknowledge that our electrocatalyst design takes inspiration from these prior works that the reviewer has mentioned, using these key ideas to design an efficient electrocatalyst for a different application: which is the direct conversion of CO2 in simulated flue gas to C2+ products.Hence, we have now cited all these relevant publications in our manuscript.

Response
The work by He et al. (Nat. Commun. 2020, 11, 3844) found that the partial current density to C2+ products could be increased in CO2/O2 mixtures as compared to pure CO2 alone, especially at the lower overpotentials.However, it is important to note that as a result of the O2 addition, even though the C2+ partial current density increases, the total FE actually decreases significantly.This is because a significant amount of current becomes consumed by ORR.For convenience, we have reproduced their results in Fig. R1 above.We observe that with addition of O2, a significant portion of the current actually becomes consumed by the ORR, such that the FE towards CO2RR products becomes very small.This observation is also consistent with our result in alkaline media (Fig. 3c), where we found that a significant portion of the current is consumed by ORR and very little FE towards CO2RR products.Importantly, we show in our work that acidic electrolyte can suppress the ORR and allow for reasonable FE values towards C2+ products.We have now cited and discussed this work in our manuscript.
The work by Lu et al. (Sci. Bull. 2019, 64, 1890-1895) was already cited and discussed in the introduction of our manuscript.The work by Li et al., (Angew. Chem. Int. Ed. 2020, 59, 10918-10923) is now cited and discussed in our manuscript.In both works, they utilized a selective CO2 mass transport strategy by coating their cobalt phthalocyanine or Sn catalysts with a selectively permeable polymer with a high CO2/O2 selectivity.As a result, they achieved a 75.9%Faradaic efficiency (FE) for CO production when a feedstock containing 5% O2 and 95% CO2 was employed.For the Sn catalyst, they generated formate with nearly 100 % selectivity and a current density of 56.7 mA cm −2 with a feedstock of 95% CO2 and 5% O2.Importantly however, they did not demonstrate if their strategy could be applied to the generation of C2+ products on Cu.Furthermore, in our case the CO2 concentration we used was much lower at only 15%.We also note that it very important to develop other strategies in addition to selective CO2 mass transport that could be simple yet effective, which we successfully demonstrated our strategy involving electrolyte selection and electrocatalyst design.This is because in future work, our strategy could be potentially combined with the selectively permeable polymer strategy to yield even better performance results.
The work by Xu et al. (Energy Environ. Sci. 2020, 13, 554-561) was already cited and discussed in our manuscript.In this work, they also employed a selective CO2 mass transport strategy by using ionomer coatings on their Cu catalysts which selectively slowed down O2 mass transport.In addition to the points raised in the previous paragraph, we note that this required high pressure conditions of 10 bar to obtain reasonable FEs to C2+ products.In our work, high pressure conditions are not required and ambient pressure was sufficient.
The work by Tian et al. (Front. Chem. 10, 915759) focused mainly on the design of CuO/CeO2 nanocomposites for selective electrochemical CO2 reduction to C2+ products.The authors explain that the addition of CeO2 to CuO helps to stabilize Cu + sites that promote C2+ product formation.In their work, pure CO2 feedstock was continuously bubbled into the electrolyte to keep the electrolyte saturated with CO2 in a membrane-free electrochemical cell.However, we note that O2 tolerance was not the main focus of their work.Rather the design of the CuO/CeO2 catalyst was the main focus.Hence, the authors did not perform a systematic study on O2 tolerance.In their system, it seemed that the O2 generated at the anode did not affect their CO2 reduction at the cathode.We reason that this is likely because: (1) pure CO2 is continuously sparged into the electrolyte, which will remove any O2 generated from the anode.(2) the current densities applied are low (5-10 mA cm -2 ) , which means that any O2 generated might be quite little.(3) the solubility of O2 in water is very low and is ~26 times lower than CO2.As a result of these factors, there might be only a very small amount of O2 dissolved in their electrolyte.We have now cited this work in our manuscript.
About reaction mechanism analysis, the authors performed DFT simulations to understand the suppression of ORR activity under acidic conditions.However, the reaction mechanism about electrochemical CO2 reduction reaction should be investigated by DFT computations, which is more key section.
We emphasize that one focus and novelty of our work is that the ORR activity can be suppressed under acidic conditions, which can enable the generation of C2+ products from simulated flue gas.This is the reason why we were motivated to perform DFT simulations to understand this suppression of ORR activity.
Therefore, this work lacks sufficient novelty to publish for the strict scientific level journal Nature Communications.

Response
Once again, we would like to sincerely thank Reviewer #1 for reviewing our manuscript.We are also grateful for these important discussion points that have been raised.We hope that we have adequately clarified the main novelty points of our manuscript and improve their opinion of our work.
The manuscript by Lum et.al. reported selective and stable electrochemical CO2 reduction (CO2R) from flue gas in acid.The idea of utilizing flue gas is more challenging yet practical for the commercialization of this technology.The authors designed tandem catalysts, Cu PTFE/Ni-N4, to achieve FE(C2+) of 46.5% at 200 mA/cm2.Various techniques, including insitu Raman and XAS, were used to characterize the mechanism and active sites.The manuscript is well-written, and the experiments are carefully designed.I, thus, recommend publishing after minor revision after addressing the following questions:

Response
We thank Reviewer #2 for their positive evaluation of our manuscript and thoughtful suggestions that have helped us improve our work.
1. Acidic electrolyte is selected by the authors at the beginning of the study.It is known that CO2RR has a much higher activity in base than acid.Can the authors explain why acid is selected for CO2RR?Besides, the solubility of K2SO4 is limited.This might be the reason for the large full-cell voltage (6.5 V).Did the authors try other salts with higher solubility?

Response
The main goal of the project was to figure out the optimal electrolyte conditions and electrocatalyst design that would enable suppression of ORR and formation of C2+ products from flue gas feedstock.Hence, when we started the project, we first experimentally screened alkaline, neutral and acidic conditions for direct conversion of simulated flue gas to C2+ products.Through this, we found that acidic conditions allowed Cu PTFE electrodes to produce the highest C2+ FE and better suppression of ORR.Therefore, although the activity in base in indeed higher under conventional pure CO2 feedstock conditions, this is not the case when flue gas is instead used as the feedstock.Hence, this is the reason why we selected acidic electrolyte at the beginning of the study.As for the exact acidic electrolyte formulation, this was based on a paper by Xie et al. (Nat. Catal., 2022, 5, 564-570), which provided us with important insights.
In addition, to explore the effect of K2SO4 solubility on the full cell voltage, we replaced this with Cs2SO4 which has a higher solubility.Hence, we conducted CO2R tests using 0.05 M H2SO4 + 1.5 M Cs2SO4 as the acidic electrolyte formulation with pure CO2 feedstock.The C2+ FE achieved a value of 87.3% at a cathodic current density of 400 mA cm -2 .As correctly highlighted by the reviewer, the full-cell voltage at 400 mA cm -2 can indeed be lowered to a smaller of 4.5 V.The applied potential at the cathode vs the Ag/AgCl reference and total-cell voltage for the various applied current densities are listed in Table R1 and Table R2 respectively (reproduced below).The corresponding FE values are shown in Fig. R2.
We note that we have already previously also used this similar strategy of replacing the K2SO4 with a higher concentration of Cs2SO4 for our studies with simulated flue gas (Fig. 3), with similar outcomes of lowering total cell-voltage and increasing the C2+ FE.Cu PTFE/Ni-N4 100 mA cm -2 3.2 V 200 mA cm -2 3.9 V 400 mA cm -2 4.5 V 600 mA cm -2 5.1 V

Response
In Fig. 2d, the highest proportion of COatop was identified to be at -2.05 V vs Ag/AgCl.On the other hand, the optimal voltage for peak C2+ FE was identified to be at -1.72 V vs Ag/AgCl.Based on previous work by Li et al. (Nature, 2020, 577, 509-513), the authors observed a volcano correlation between the COatop to CObridge ratio to the C2+ FE.Similar in our case, we postulate that a similar situation could be occurring, with an optimal COatop to CObridge ratio occurring at -1.72 V vs Ag/AgCl, resulting in peak C2+ FE at this potential.
Besides CO adsorption, we reasoned that the intrinsic HER activity of the catalyst could also be an important factor that affects the C2+ FE.This is because the HER is a competing parasitic reaction during CO2R.As a comparison, the HER performance of Cu PTFE/Ni-N4 and Cu PTFE was tested in 0.5 M K2SO4 + 0.05 M H2SO4.As shown in Fig. R3, Cu PTFE/Ni-N4 shows lower HER activity as compared to Cu PTFE.Specifically, Cu PTFE/Ni-N4 has a higher overpotential of 362 mV to reach a current density of 10 mA cm -2 , as compared to only 292 mV with Cu PTFE.This is consistent with our CO2R FE results, where Cu PTFE/Ni-N4 exhibited a lower HER FE as compared to Cu PTFE.As for the *CO population, we only observed the presence of COatop with simulated flue gas (Fig. S42b).This situation appears to differ from the behaviour observed in pure CO2 gas, where CObridge was also observed.We speculate that the absence of CObridge could be due to the competitive adsorption of ORR intermediates on the catalyst surface.

Response
For our catalyst, we sputtered 5 nm of Cu to construct each layer, which would most likely cover some of the Ni-N4 sites.However, in our design principle, we reasoned that if this layer was too thin, there would be an inadequate amount of Cu active sites to further convert to CO generated by the Ni-N4 catalyst.On the other hand, if this Cu layer was too thick then too many of the Ni-N4 active sites would be covered and CO2 to CO conversion would become compromised, resulting in a lower C2+ FE.Hence in our optimization process, we had prepared electrodes with varying thicknesses of sputtered Cu layers (2, 5 and 10 nm).As illustrated in Fig. S20, we find that the total C2+ FEs are 47.0%, 75.2% and 55.9% for 2, 5 and 10 nm Cu respectively.Hence, through this empirical process, we found that the optimal Cu thickness was 5 nm, and this is why this value was chosen for our fabrication strategy.We found that Cu PTFE/Ni-N4 does indeed give a higher ORR activity compared to Cu PTFE in alkaline media (Fig. 3a).However more importantly, this trend is reversed in acidic media, where Cu PTFE/Ni-N4 has a lower ORR activity compared to Cu PTFE.While we do not know the exact reasons for this, we note that the acidic condition is more relevant to our work, for enabling production of C2+ products from simulated flue gas and suppressing ORR activity.

Response
The corresponding typos have been corrected in the revised manuscript.
Once again, we sincerely thank Reviewer #2 for their helpful comments.
Reviewer #3 (Remarks to the Author): In this manuscript, the authors describe a strategy for the generation of multicarbon products from simulated flue gas, where acidic electrolyte was used to significantly suppress ORR on a Cu PTFE/Ni-N4 electrode.The authors achieve FEs toward multicarbon products of 82.4 % and 46.5% in pure CO¬2 gas and in simulated flue gas respectively.This was supported theoretically using DFT simulations where they found increases in the free energy change of the rate-determining steps for ORR on Cu and Ni-N4 in acidic media.This manuscript is suitable for publication in Nature Communications after appropriate revision, with detailed comments as given below:

Response
We thank Reviewer #3 for taking the time to evaluate our manuscript and providing helpful suggestions to improve our manuscript.
1.More statistical analysis of the Ni-N4¬ size should be provided, which in the ideal case should yield a distribution close to a Gaussian shape.

Response
We have conducted a re-evaluation of the particle size distribution.The updated size distribution outcomes are presented as below: 2.More information should be provided regarding the computational methods, such as why the top two layers of Cu (111) were chosen to move freely, the criteria for selecting K-points, and whether spin polarization was considered.

Response
The reason why the top two layers of Cu (111) were chosen to move freely is because the interactions between the adsorbates and the two lower layers are negligible compared to that between the adsorbates and the top two layers.Fixing the two lower layers can reduce computational demand and expedite the computation process.Therefore, we used a 4-layer Cu (111) model with the two upper layers relaxed and the two lower layers fixed.
Choosing an appropriate K-point grid density is crucial.Higher density yields more accurate results but at a higher computational cost.In this work, we start with a lower K-point density and gradually increase it until the results converge, with no significant changes.To ensure convergence of the k-point grid size, we evaluated the overall energy of Cu ( 111) from 1 1 1 to the 8 8 1 k-point grid.The grid size was increased to 4 4 1, achieving convergence.Therefore, in the Cu ( 111) system, the k-point grid is 4 4 1 (Fig. R4).
In addition, the missing computational parameters have been added in the revised manuscript.

Response
In our optimization process, we empirically found 3 layers to give the optimal performance for C2+ product FE.In Figure S21 we show the CO2R performance obtained when an additional 4 th layer of Ni-N4 + sputtered Cu layer is added (Cu PTFE/Ni-N4-4).We found that with 4 layers, the total C2+ FE decreases to a value 61.3%.This is the reason why we selected 3 layers, which gave the best performance results (Fig. S21).We reason that this is because too many layers could result in CO2 mass transport limitations.
6. Authors should check through the manuscript for mistakes, for example there are some errors in subscripts/superscripts.

Response
The corresponding errors have been corrected in the revised manuscript.
Once again, we would like to thank the reviewer for providing helpful suggestions to improve our work.
are also present at the more negative potentials.This suggests that the local pH is no longer acidic, due to consumption of protons and generation of hydroxide anions.
It is well known that in an acidic solution, the critical pH for the hydrolysis of carbon dioxide (CO2) depends primarily on the acid-base equilibrium in water, particularly the equilibrium reaction between CO2 and water.CO2 can undergo hydrolysis with water to form carbonic acid (H2CO3), which can then further dissociate into bicarbonate ions (HCO3 -) and protons (H⁺): CO2 + H2O ↔ H2CO3 ↔ HCO3 -+ H + In this reaction, the critical pH in an acidic solution typically refers to the pH at which the concentrations of H⁺ ions and HCO3 -ions are equal.This corresponds to the balance between carbonic acid and its dissociation products.At room temperature, this critical pH value is usually around 6.35.Additionally, CO2 typically does not undergo hydrolysis into carbonic acid when the pH is less than approximately 4.5 (J. Geosci. Educ., 2002, 50, 357-362).This is because at lower pH conditions, the increased concentration of hydrogen ions in the water promotes the equilibrium reaction between bicarbonate ions and carbon dioxide to shift to the left, resulting in more CO2 formation rather than the formation of carbonic acid.
Based on this analysis and our in-situ Raman spectroscopy results, we thus concluded that the local pH of our electrode under CO2R conditions should be below 4.5 and hence remains acidic when an electrolyte of 0.05M H2SO4 + 0.5 M K2SO4 (bulk pH 1.7) is used.On the other hand, when the bulk pH was adjusted to 2.2, the local pH can no longer be maintained in an acidic region under CO2R conditions.

Response
According to this suggestion, we have fitted the high-resolution XPS spectrum and updated Fig. S4, S9, S14, S23 (all reproduced below).For Cu 2p, it is well-known that Cu + and Cu 0 have very similar positions, hence it is difficult to deconvolute their individual contributions.Hence, we do not provide the relative proportions of Cu 0 :Cu + :Cu 2+ .For the Ni-N4 catalyst, we have deconvoluted the relative contributions of pyridinic, graphitic, oxidized and pyrrolic N (Fig. S9b, Fig. S14b and Fig. S23b).

Response
In CO2R experiments, the overall FE is a summation of all detected gas and liquid products.
There is no definitive guideline in the field, however total reported FEs should ideally be at least >90% for reliable results.This is because minor products could be generated, which were not detected or analyzed.In addition, some liquid products could also be more volatile and hence losses could easily occur.Products such as ethanol could also migrate to the anode and oxidize, further contributing to losses.In addition, there could also be experimental and human errors which contribute to the overall FE not being a 100%.Hence typically, results are reported with error bars indicating the standard deviation from three independent measurements to ensure reliable results.In our work, our total FE generally lies close to a 100%.5.The author produced the EIS spectrum in Figure S18, but there is no significant change in the Rct value of the three catalysts to claim that the Ni-N4 catalyst possesses faster kinetics.Thus, the claim is ambiguous.

Response
We have carefully considered this statement again and agree with the reviewer.To avoid any misunderstanding, we have removed the sentence related to "faster kinetics".
6.The authors performed DFT simulations on ORR activity, however in acidic conditions a DFT calculation about CO2RR with hydrogen evolution path will give more insights.

Response
The reviewer has raised a good point regarding the HER activity, which tends to compete with CO2R and hence is a parasitic side reaction.Although the ORR FE becomes suppressed in acidic electrolyte, we observed that the HER FE actually becomes slightly increased in acidic electrolyte as compared to alkaline electrolyte in our simulated flue gas experiments (Fig. 3c and Fig. S36).For example, at a current density of 200 mA cm -2 , the HER FE for Cu PTFE/Ni-N4 has a value of 20.6% in alkaline electrolyte and 30.6% in acidic electrolyte.This is expected because HER in acidic electrolyte directly consumes protons and hence avoids the water dissociation step which is known to be rate-limiting in alkaline HER (Adv. Funct. Mater., 2021, 31, 2101578) (Energy Environ. Sci., 2021, 14, 5228-5259) (Adv. Energy Mater., 2019, 9, 1901333).As a result, performing CO2R in acidic media without alkali metal cations is known to result in nearly 100% FE for HER (Science, 2021(Science, , 372, 1074(Science, -1078)).With the addition of alkali metal cations, Huang et al. showed that the HER activity could be suppressed and hence reasonable FEs towards C2+ products could be achieved in acidic electrolyte through addition of a high concentration of alkali metal cations (Science, 2021(Science, , 372, 1074(Science, -1078)), which we have also done in this work.
Following the reviewer's suggestion, we had initially considered to perform DFT calculations to understand this HER activity suppression effect by these alkali metal cations.However, upon reviewing the literature we found that explaining this effect cannot be adequately accomplished through DFT calculations.Instead, this effect might be explained better through Poisson-Nernst-Planck (PNP) simulations (Nat. Catal., 2022, 5, 268-276).For instance, work by Gu et al. (Nat. Catal., 2022, 5, 268-276) found that the reason for reduced HER activity in their acidic electrolyte is due to the high concentration of hydrated alkali cations that they employed (K + cations).Using a Poisson-Nernst-Planck (PNP) simulation model, the authors found that hydrated alkali cations physisorbed on the cathode modify the distribution of electric field in the double layer, which then impedes HER by suppressing the migration of hydronium ions.Similarly, in our case the acidic electrolyte employed contains a high concentration of alkali metal cations (K + and Cs + ).Hence, we reasoned that a similar effect likely operates in our system to suppress the HER activity for operating in both pure CO2 and simulated flue gas feedstock with acidic electrolyte.
7. What is the practical relevance of this single atom catalyst?How feasible is it to upscale it and bring to practical use beyond the lab scale study?

Response
The practical relevance of single atom catalysts (SACs) is the potential to achieve near 100% atomic utilization, meaning that every single metal atom in the catalyst could potentially serve as an active site (Electrochem.Energ. Rev., 2019, 2, 539-573) (Nat. Rev. Chem., 2018, 2, 65-81) (Chem. 2019, 5, 2733-2739).This is in contrast to bulk films or nanoparticles, where metallic atoms that are buried in the subsurface typically do not participate in facilitating the reaction of interest and are hence inactive.This high atomic utilization is especially important for noble metals such as Pt, which tend to be expensive and scarce.In addition to this, SACs tend to have very different catalytic properties from their bulk counterparts.For instance, bulk Ni is inactive for CO2R, but Ni SAC can selectively convert CO2 to CO.
Our catalyst synthesis method is facile, consisting of a hydrothermal procedure, followed by metal impregnation and thermal annealing and could potentially be amenable to scale up.Although we did not attempt to scale up our synthesis process, we turn to the literature where studies on SAC synthesis scale up have been performed.For instance, Yang et al. (Nat. Commun., 2019, 10, 4585) introduced a universal and robust ligand-mediated approach for synthesizing M-SACs with high metal content.This method is versatile and applicable to the synthesis of M-SACs containing first-, second-, and third-row transition metals on carbon supports.Furthermore, it leverages commercially available conductive carbons as a support and facilitates large-scale SAC production, reaching the kilogram scale.Additionally, Sun et al. (Nat. Commun., 2023, 14, 1599) reported a microwave-assisted strategy for fabricating coordinatively unsaturated metal-nitrogen sites doped within defective carbon nanotubes (Fe, Co, or Ni-CNTs-MW).This simple synthetic approach is universal for gram-scale production within approximately 2 minutes.These studies therefore indicate the potential for further scaleup of SACs for practical deployment.
More relevant to CO2R, Zheng et al. (Joule, 2019, 3, 265-278) synthesized single-atom Ni catalysts for CO2 to CO conversion and deployed this for use in a large area 100 cm 2 membrane electrode assembly CO2 electrolyzer system.They demonstrated long-term electrolysis under a full-cell voltage of 2.8 V and a current of 8 A. At the same time, the CO selectivity maintained above 90% over the course of 6 hours of continuous operation.Hence, this study indicates that these SACs could potentially be synthesized and deployed on a larger scale.Based on these studies, we believe that there is a promising route to bring SACs to practical use beyond lab scale studies.
8. What about testing with real flue gas which will give a more realistic case?

Response
Real flue gas consists of non-negligible amounts of impurities such as SOx and NOx.For the case of SOx, this is known to contaminate the Cu surface (J.Am.Chem. Soc., 2019, 141, 25, 9902-9909) and cause an irreversible switch of the reaction pathway entirely to formate production.As for NOx, previous work (Nat. Commun., 2020, 11, 5856) has shown that although its effect is reversible, NOx reduction can compete with CO2 reduction and hence can consume a good portion of the current.In this work, the focus was to overcome the dual challenges of low CO2 concentration and the competing ORR.Hence, we employed simulated flue gas to study this in a systematic and reproducible way.In future work, we are currently indeed actively researching on ways to address the SOx/NOx issue.Once this is resolved, directly employing real flue gas would become possible.In addition, we are currently in active talks with industry partners who can supply us with real flue gas.
Once again, we would like to sincerely thank the reviewer for their constructive comments which have helped us improve the quality of our work.
Once again, we thank the editor for handling our manuscript and the reviewers for their comments.Changes within the manuscript and supporting information are highlighted in yellow.Below is a response to the reviewer comments, which are written in blue font.
Reviewer #2 (Remarks to the Author): All my concerns are properly addressed.

Response
We are thankful to Reviewer #2 for their constructive advice and suggestions.
Reviewer #3 (Remarks to the Author): I can see the authors have conducted additional research and discussions to address the concerns properly.Thus I would recommend acceptance at this stage.

Response
We are grateful to Reviewer #3 for their time and helpful comments.
Reviewer #4 (Remarks to the Author): In the revised version, the authors seem to respond appropriately to what we have asked.The manuscript, SI, and experimental data were addressed point-by-point.The defects in the main text, SI, figure captions, and figures were modified.

Response
We would like to thank Reviewer #4 for taking the time to evaluate our revised manuscript.
1.However, the discussion about XPS seems to be weak and the fitted curves in Figure S14 and S23 is not appropriate.

Response
We have refitted and updated the XPS data in Fig. S14 and S23 (reproduced below).Specifically, the Cu 2p3/2 XPS fitting curves revealed that Cu was mainly in the oxidized state in Cu PTFE/Ni-N4.Our Raman spectroscopy results (Fig. S5) also corroborates the presence of oxidized Cu on the sample surface.Since the Cu films are prepared by sputtering, they should be in the metallic form in the pristine as prepared condition.We note that the XPS data are included as part of the catalyst characterization process.However, due to oxidation of the catalyst in ambient air, there is no clear relevance between this XPS data and explaining the catalyst activity/performance.For this reason, we did not provide any in-depth discussion of the XPS results in the previous manuscript versions.2. We are still concerned about the DFT calculation.The authors keep the assumption that the missing FE can be assigned to ORR which cannot be true as there will be a hydrogen pathway.

Response
We clarify that we actually do indeed quantify the FE towards the hydrogen pathway (hydrogen evolution reaction HER) and report these results together with the CO2 reduction products.For instance, in Fig. 3c (reproduced below) the HER FE is shown in light grey and the ORR FE is shown in dark grey.Exact FE values and full results are also provided in Tables S14-S40.In our CO2R experiments with flue gas, both HER and ORR serve as competing reactions.We are able to measure the HER FE using gas chromatography.However, it is challenging to determine the ORR FE because we operate in aqueous electrolyte and the product generated is water.Therefore, we have to assume that the missing FE must be attributed to ORR.We note that this assumption has also been employed in recent publications (J.Am.Chem. Soc. 2023, 145(48), 25933-25937) (Energy Environ. Sci. 2020, 13, 554-561).
According to this suggestion, we have also performed additional DFT calculations of the hydrogen pathway.Commun. 2022Commun. , 13, 1189)).Based on our DFT calculations, we found that the Ni-N4 system exhibits energy barriers of 2.56 eV in alkaline media and 1.57 eV in acidic media for HER (Fig. S32-S33).As for Cu (111), energy barriers of 0.23 eV in alkaline media and 0.12 eV in acidic media (Fig. S32-33) were determined.These results suggest that HER is easier in acidic media as compared to alkaline media and is consistent with expectations (Adv.Energy Mater. 2020, 10, 2002260) (Adv.Mater. 2021, 33, 2007894).This is also consistent with our FE data where the HER FE for Cu PTFE/Ni-N4 was found to be 20.6% in alkaline electrolyte (Fig. S37) and 30.6% in acidic electrolyte (Fig. S39) at a current density of 200 mA cm -2 .However, we note that even though HER activity is significantly higher in acidic media, the presence of a high concentration of alkali metal cations (e.g.K + ) can help to suppress the HER activity and promote CO2 reduction.This was previously demonstrated by Huang et al. (Science, 2021(Science, , 372, 1074(Science, -1078) ) and also successfully employed in our work.3.However, we are also concerned about the originality of this work because there are previous reports on similar work.The authors need to clarify this point to emphasize this work's idea and strong point.

Response
We would like to emphasize that the novelty of the work is on identifying an electrolyte composition and electrocatalyst system combination that can enable the generation of multicarbon (C2+) products directly from simulated flue gas with reasonable FEs.In this process, we needed to solve the two main challenges involved, which is the low CO2 concentration (15%) and suppressing the competing oxygen reduction reaction.For the first time, we have successfully demonstrated in this work that acidic electrolyte in combination with a Cu PTFE/Ni-N4 tandem electrocatalyst system can solve these challenges.
To the best of our knowledge, there is only work by Xu et al. (Energy Environ. Sci. 2020, 13, 554-561) that the direct utilization of simulated flue gas (N2/CO2/O2 mixture) for generation of multicarbon products.In this work, they employed a selective CO2 mass transport strategy by using ionomer coatings on their Cu catalysts which selectively slowed down O2 mass transport.However, we note that this required high pressure conditions of 10 bar to obtain reasonable FEs to C2+ products.In our work, high pressure conditions are not required and ambient pressure was sufficient.Furthermore, our simple yet effective strategy involving electrolyte selection and electrocatalyst design is completely different and novel.
Below is a comparison of our work with previous literature involving electrochemical CO2 reduction with mixtures of CO2/O2.
Work by He et al. (Nat. Commun. 2020, 11, 3844) found that the partial current density to C2+ products could be increased in CO2/O2 mixtures as compared to pure CO2 alone, especially at the lower overpotentials.However, it is important to note that as a result of the O2 addition, even though the C2+ partial current density increases, the total FE actually decreases significantly.This is because a significant amount of current becomes consumed by ORR with addition of O2, such that the FE towards CO2R products becomes very small.This observation is also consistent with our result in alkaline media (Fig. 3c), where we found that a significant portion of the current is consumed by ORR and very little FE towards CO2R products.Importantly, we show in our work that acidic electrolyte can suppress the ORR and allow for reasonable FE values towards C2+ products.
Work by Lu et al. (Sci. Bull. 2019, 64, 1890-1895) and Li et al., (Angew. Chem. Int. Ed. 2020, 59, 1091810923) utilized a selective CO2 mass transport strategy by coating their cobalt phthalocyanine or Sn catalysts with a selectively permeable polymer with a high CO2/O2 selectivity.As a result, they achieved a 75.9%Faradaic efficiency (FE) for CO production when a feedstock containing 5% O2 and 95% CO2 was employed.For the Sn catalyst, they generated formate with nearly 100 % selectivity and a current density of 56.7 mA cm −2 with a feedstock of 95% CO2 and 5% O2.Importantly however, they did not demonstrate if their strategy could be applied to the generation of C2+ products on Cu.Furthermore, in our case the CO2 concentration we used was much lower at only 15%.We also note that it very important to develop other strategies in addition to selective CO2 mass transport that could be simple yet effective, which we successfully demonstrated our strategy involving electrolyte selection and electrocatalyst design.This is because in future work, our strategy could be potentially combined with the selectively permeable polymer strategy to yield even better results.
These works are cited and discussed in our manuscript.
Once again, we are grateful to the reviewer for these helpful suggestions to improve our work.

REVIEWERS' COMMENTS
Reviewer #4 (Remarks to the Author): Pleased to read this revised version of the manuscript and notice that the authors have carried out the revisions suggested to them.Indeed, the revised manuscript looks much better than the original submission.Also, the authors have answered very clearly all the concerns raised by me as and other reviewers as well.For my comments, I have no further remarks to make on this manuscript and I am satisfied with the current revised version.
Fig S4, Fig S14 should be assigned.The survey spectrum shows some other peaks that should be identified.The high-resolution spectrum needs to be fitted properly to find out the oxidation states.4. In Fig S6, and Fig 2 why the overall FE is not 100%, if it's not 100%, does the author expect any other products?Then in Figure

Figure R2 |
Figure R2 | FE results of Cu PTFE/Ni-N4 in 0.05 M H2SO4 + 1.5 M Cs2SO4 at various applied cathodic current densities with pure CO2 feedstock.2. The in-situ Raman studies are very informative to obtain insights into the reaction mechanism.CO(atop) is more active than CO(hollow+bridge) for C2+ formation.In Fig 2d, the highest portion of COatop was found at -2.05V, while the faradaic efficiency of C2+ peaked at -1.72 V. Did the authors consider other reasons besides CO adsorption for the selectivity of

Figure R3 |
Figure R3 | LSV curve of Cu PTFE/Ni-N4 and Cu PTFE in 0.5 M K2SO4 + 0.05 M H2SO4.An in-situ Raman spectroscopy study of Cu PTFE/Ni-N4 was also performed in 0.05 M H2SO4 + 1.5 M Cs2SO4 electrolyte with simulated flue gas.This aimed to provide insights into the CO2R mechanism under the influence of introduced O2.As shown in Fig.S42a, the peaks at 970, and 1040 cm -1 represent the O-O stretching vibration of O2 (Nat.Commun.2016, 7, 12440) and OH species (J.Am.Chem.Soc., 2020, 142(2), 715-719) adsorbed on Cu PTFE/Ni-N4.The existence of *O2 and *OH species unambiguously confirms the occurrence of ORR on the catalyst surface.As for the *CO population, we only observed the presence of COatop with simulated flue gas (Fig.S42b).This situation appears to differ from the behaviour observed in pure CO2 gas, where CObridge was also observed.We speculate that the absence of CObridge could be due to the competitive adsorption of ORR intermediates on the catalyst surface.

Figure S42 |
Figure S42 | (a) Potential resolved in-situ Raman spectroscopy of Cu PTFE/Ni-N4 in 0.05 M H2SO4 + 1.5 M Cs2SO4 with simulated flue gas.(b) Raman heatmap of Cu PTFE/Ni-N4 in the 2000-2120 cm -1 region, showing the dynamic behavior of *CO. 3. The Cu PTFE/Ni-N4 electrode is fabricated through a layer-by-layer strategy.Will the active site of Ni be covered by Cu, especially when the concentration of Ni is very low?Is there any systematic design strategy to ensure the exposure of the Ni active sites to proceed for tandem catalysis as suggested?In Fig 3.a, the Cu-PTFE/Ni-N4 showed a higher ORR activity than Cu-PTFE alone.What is the rationale for choosing Ni-N4?Did the authors consider choosing other catalysts with poorer ORR activity?

Figure S20 |
Figure S20 | CO2R performance in acidic electrolyte.The FE results for Cu PTFE/Ni-N4 with different thickness (2, 5 and 10 nm) of copper layer.The black squares represent the total C2+ FE for each catalyst condition.
Minor comments: Typos in Line 222.I think the authors meant Fig 2d rather than Fig 2f.

Figure 1c |
Figure 1c | TEM images of Ni-N4, consisting of Ni single-atoms hosted on a carbon support.

Figure S39 |
Figure S39 | FE results of Cu PTFE in 0.05 M H2SO4 + 0.5 M Cs2SO4 at different current densities with simulated flue gas.

Figure S40 |
Figure S40 | FE results of Cu PTFE in 0.05 M H2SO4 + 1.5 M Cs2SO4 at different current densities with simulated flue gas.4. In Figure S20, the authors claim that the CO2R performance of Cu PTFE/Ni-N4 surpasses that of Cu PTFE/Ni-N4-1 and Cu PTFE/Ni-N4-2.What will happen to the performance if more layers are added to the catalyst system?

Figure S21 |
Figure S21 | CO2R FE results in 0.5 M K2SO4 + 0.05 M H2SO4 for Cu PTFE/Ni-N4-1, Cu PTFE/Ni-N4-2, Cu PTFE/Ni-N4, and Cu PTFE/Ni-N4-4 at 200 mA cm -2 . 5. Please elucidate the procedure employed by the authors to determine the area/proportion of *COhollow, *CObridge, and *COatop, as well as how they validated the ratio of *CO(hollow + bridge) to *COatop?Is this method an original development, or has it been previously established?If this is not a new method, kindly provide additional details and cite the relevant research articles.

Figure S30 |
Figure S30 | Raman spectra recorded at various pH value.(a) bulk: pH =1.7 (b) bulk: pH =2.2.HCO3 − and CO3 2− peaks are recorded at 1012 and 1064 cm -1 .Voltages are reported vs Ag/AgCl.OCP stands for open-circuit potential, where no current is passing through the system.3. The XPS spectrum in Fig S4, Fig S14 should be assigned.The survey spectrum shows some other peaks that should be identified.The high-resolution spectrum needs to be fitted properly to find out the oxidation states.
Fig. R5 are examples taken from the literature, which illustrates that such variations are common in CO2R tests.

Figure 3c |
Figure 3c | Product FE for Cu PTFE in 1 M KOH and Cu PTFE/Ni-N4 in 0.05 M H2SO4 + 1.5 M Cs2SO4 under different current densities.

Figure S32 |
Figure S32 | Alkaline HER free energy diagram of (a) Ni-N4 and (b) Cu (111).The red dotted line represents the activation energy barrier for the rate-determining step.

Figure S33 |
Figure S33 | Reaction pathway for proton reduction to hydrogen on Ni-N4 and Cu (111), involving *H as the intermediate.The red dotted line represents the free energy change for the rate-determining step.

Table R1 |
Potential vs Ag/AgCl for CO2R in 0.05 M H2SO4+1.5 M Cs2SO4 for Cu PTFE/Ni-N4 at various applied cathodic current densities with pure CO2 feedstock.

Table R2 |
Full-cell voltage for CO2R in 0.05 M H2SO4+1.5 M Cs2SO4 of Cu PTFE/Ni-N4 at various applied cathodic current densities with pure CO2 feedstock.
These oxidized forms of Cu are a result of oxidation of the exposed Cu surface in ambient air, which reduce to its metallic form upon application of a cathodic potential during CO2 reduction (Angew.Chem.Int.Ed.2018, 57, 551-554) (Appl.Surf.Sci.2018,255,2730-2734)(ACS Appl.Mater.Interfaces 2018, 10, 10, 8574-8584).Discussion of the XPS results has now been included in the manuscript.